The measurement of fluid properties is important in many industrial and consumer machines and processes. While applicable to many fields, the measurement of fluid properties is of significant concern for lubrication and power transfer equipment, which is based on mineral oils and related synthetic compounds. Loss of functionality of these fluids results in premature aging of the associated equipment and, sometimes, in catastrophic failure.
Presently, the characterization of such fluids is primarily accomplished using laboratory analysis of fluid samples. There is a growing desire to place sensors for continuous monitoring directly into the equipment.
In U.S. Pat. Nos. 5,708,191, 5,886,250, 6,082,180 and 6,082,181, Greenwood et al. present a family of densitometer sensor designs that employ input and output transducers to measure changes in reflected signal strength of acoustic waves as they reflect near a critical angle of incidence. These viscometers measure either viscosity-density product or elasticity-density product based on the reflection of acoustic waves from the liquid-loaded face of a solid material supporting the transducers. The sensors measure reflection coefficients of the wave from solid-liquid boundaries upon a few reflection events of a pulsed or continuous-wave signal. Such methods offer less sensitivity and resolution of the measured quantity than do resonant, multi-reflective, or surface generated acoustic wave devices. The latter enjoy higher sensitivity from the continuous interaction of their acoustic waves with the solid-liquid interface. The discrete-reflection methods do not enjoy the simplicity of manufacture or operation of a continuous acoustic wave interaction surface, nor do they provide similar sensitivity or resolution. The discrete reflection methods assume a fixed elastic modulus (for the more common compressional wave version) or viscosity (for the less common shear wave version) in order to extract density information from the measured response of the sensor.
Measuring the density-viscosity product is done by immersing a resonator manufactured on a piezoelectric substrate and supporting a transverse shear mode of resonance, typically a disc of quartz crystal of AT cut or langasite of approximately Y-cut, into the liquid and measuring the shift in resonant frequency or the loss of power at resonance. Further known is the use of two-port devices based on multi-pole resonators using shear wave acoustic modes, such as the SH-SAW (Shear Horizontal—Surface Acoustical Wave), SHAPM (Shear Horizontal Acoustical Plate Mode), MPS (Monolithic Piezoelectric Sensor) e.g. as described in U.S. Pat. No. 6,033,852, issued Mar. 7, 2000 to Andle et al.
U.S. Pat. No. 5,741,961 and U.S. Pat. No. 5,798,452 to Martin et al. disclosed a method in which two acoustic wave sensors having different surface treatment exhibit essentially identical responses to viscosity-density product but differing responses to the density. A reference sensor provides data on the product of viscosity and density and employs a smooth surface. The second sensor has an intentionally roughened surface, typically having grooves or pits in its surface for trapping a certain volume of fluid. The added mass creates a finite frequency shift with little or no power loss in addition to the power loss and frequency shift of the viscously entrained liquid. The density-viscosity product is available via the common-mode frequency shift. While this method is attractive, it incurs difficulties in sensor-to-sensor reproducibility, particularly when the two sensors are manufactured on different substrates.
The addition of such grooves to one of a pair of shear horizontal surface acoustic wave (SH-SAW, also known as Love Wave, surface transverse wave, and the like) sensors is also disclosed (Herrmann et al., U.S. Pat. No. 6,543,274), and the extension of this approach to SHAPM sensors is contemplated herein. This method offers higher frequency, smaller size, and improved sensitivity at the expense of manufacturing complexity and available dynamic range. However Herrmann still finds the use of two completely separate but identical (other than the corrugations) sensor elements necessary for measuring both parameters, and therefore it does not overcome the sensor-to-sensor limitations of the Martin device. It also incurs less accurate viscosity measurement due to the higher operating frequency, which exacerbates shear thinning and Maxwellian viscoelastic issues that are known in the field of rheology.
In a U.S. Pat. No. 7,007,546, issued Mar. 7, 2006 titled “Measurement, Compensation and Control of Equivalent Shear Rate in Acoustic Wave Sensors” (which is incorporated herein by reference in its entirety), the inventor of the present application disclosed a method for measuring viscosity and shear rate at which the measurement is performed by utilizing an acoustic wave sensor, and calculating the shear rate as a function of the characteristic rate of fluid movement in response to a given power transmitted to a fluid, and the viscosity of the fluid. The acoustic wave device has a characteristic relationship between input power, output power, and an acoustic wave amplitude at a selected region between the input and output transducer. The acoustic wave device is coupled to the measured fluid. A predetermined power level Pin of a harmonic signal is applied to an input transducer, to impart an acoustic wave at the selected region. Output power level Pout is measured at the output transducer. Using the characteristic relationship, and the input and output power levels, the amplitude of the average acoustic wave imparted to the fluid is calculated. Measuring the viscosity of the fluid to obtain a measured viscosity at the selected region, allows the calculation of the shear rate of the fluid at the selected region, by using the frequency, the viscosity measurement, and the acoustic wave amplitude. This invention may be beneficially used with the present invention as explained below.
In U.S. patent application Ser. No. 10/597,487 filed Feb. 14, 2007 and published as US -2007-0144240-A1, the applicant described a two-port, two-pole coupled resonator with a textured entrapment layer in contact with a fluid to be measured, such as a liquid or a gas, which allows measurement of viscosity and density of the fluid. However, the manufacture of a textured entrapment layer adds complexity to such a device. Nonetheless, the structures and methods disclosed in the 10/597,487 application may be practiced in conjunction with the present invention. It is noted that the incorporation of a textured surface is not necessary to embody aspects of the present invention.
There is therefore a clear advantage for sensors and measurement methods that will allow measurement of as many parameters of a fluid as possible, integrated into a single device. While such devices were contemplated, they suffer from different disadvantages such as manufacturing complexity, unpredictability, low accuracy and the like. It is a goal of the present invention to provide a sensor capable of deducing at least two of the three parameters—viscosity, density, and elastic modulus, when the third parameter is known or assumed. It is a further goal of the present invention to provide a system and method that will perform such measurements using the sensor.